Factors affecting the ultimate capabilities of high speed avalanche photodiodes and a review of the state-of-the-art

The ultimate limitations to the performance of existing and new photodiode structures, which are imposed by both theoretical considerations (noise factor, amplitude distribution of gains, gain-bandwidth product, etc.) and practical limitations (choice of materials, device uniformity, difficulty of designing high speed structures in which most of the light is absorbed in a region of the depletion layer such that the carriers being multiplied are of the more strongly ionizing type, etc.), are examined. It is shown that for a properly designed structure, a useful figure of merit for high-speed (high-gain) operation is given by η/k^1/2, where η is the quantum efficiency and k is the effective ratio of the ionization coefficient of the weakly ionizing carrier to that of the strongly ionizing carrier. It is shown further that for various reasons the prospects for achieving low-noise avalanche photodiodes having gain-bandwidth products much above 200 GHz are not too bright.